When conventional screws are inserted into materials, remnants or shavings can be produced that are displaced toward the top of the screw insertion point. The remnants can remain on the surface of the material at or near the screw insertion point, requiring removal by sanding or other methods. Remnants and otherwise displaced composite material may also result in “mushrooming,” where a mound or bump of raised material surrounds the perimeter of the screw head. The mushroom then must be hammered or forced back down to maintain a flush surface.
Although pre-drilling holes for the screw may reduce remnants on the surface and/or mushrooming, this technique is laborious and time-consuming.
Several patents disclose dual threaded screws for securing a workpiece. For instance, U.S. Pat. No. 6,666,638, issued to Craven, discloses a deck screw having multiple threaded sections. The upper threaded portion has a thread pitch that is lesser than the lower threaded portion and the upper threaded portion has an outer thread diameter that is greater than the outer diameter of the lower threaded portion.
U.S. Pat. No. 6,966,737, issued to McGovern, discloses a deck screw suitable for use with composite lumber having an upper threaded portion and a lower threaded portion. The shaft in the upper threaded portion has a larger diameter than the shaft in the lower threaded portion, and the upper threaded portion has more threads per unit length than the lower threaded portion.
U.S. Pat. No. 6,000,892, issued to Takasaki, discloses a wood screw having an upper threaded portion and a lower threaded portion. The upper threaded portion has a thread angle greater than that of the lower threaded portion, and the upper threaded portion has a lead angle smaller than the lower threaded portion.
U.S. patent application Ser. No. 2004/0141827 discloses a self-tapping screw for composite materials having an upper threaded portion and a lower threaded portion separated by a thread-free section. The threads of the upper threaded portion have an opposite pitch from the lower threaded portion.
U.S. Pat. No. 6,074,149, issued to Habermehl et al., discloses a threaded fastener with an upper threaded portion and a lower threaded portion. The lower threaded portion has threads of a larger size and greater cross-sectional area than the threads of the upper threaded portion.
While these configurations have their benefits, the need exists for an improved dual threaded screw or fastener having enhanced performance yet providing maximum opportunity for retaining remnant material below the surface of the workpiece.